# This is a sample Python script.
import math
str =input("请输入字符串")
def reverse(s):
    if s=="":
        return s
    else:
        return reverse(s[1:]) + s[0]
def letters(s):
    i=0
    if(97< ord(s[i])<122):
        return s[i]
    else:
        i=i+1
        return letters(s[i])
print(letters(str))